1. Field of the Invention
The present invention relates to a sheet handling apparatus for use in combination with an image forming apparatus such as copying machine, printer, laser beam printer or the like, and more particularly to a sheet handling apparatus including a sheet post-processing device to be connected to said image forming apparatus for stacking the sheets discharged from said apparatus and collating, stapling or punching said sheets, or an intermediate tray provided in the image forming apparatus for temporarily stacking sheets, bearing image on a face, for image formation on the other face or multiple image formation on the same face.
2. Related Background Art
There is already known, as disclosed in the U.S. Pat. No. 4,717,134, a sheet processing apparatus to be connected to a copying machine and capable of transporting the sheets discharged from the copying machine onto a processing tray, then registing the ends of the sheets stacked on said tray and stapling said sheet, and transporting the stapled bundle of sheets from the processing tray onto a storage tray. Also decurling and registration of sheets are conducted in an intermediate tray in the copying machine.
In such sheet processing apparatus, during the processing of sheets on the processing tray, the next sheet cannot be sent to said tray and this fact gives rise to a waste in time, leading to the deteriorated productivity in the entire system.
For resolving this drawback, the Japanese Laid-open Patent No. 60-204570 proposed a structure of temporarily storing the sheets, discharged from the copying machine, in a storage tray, and feeding the tray stored in the storage tray in response to the command from the sheet post-processing apparatus. However, such structure becomes inevitably large and expensive as there is required means for separating and feeding the sheets, stacked in the storage tray, in the order of storage namely one by one from the bottom to the sheet post-processing apparatus.
Also in a finishing apparatus disclosed in the Japanese Patent Publication No. 62-22145, there is employed a structure of stopping a pair of transport rollers provided in a transport path for guiding the sheets to a stapling tray, stopping the first sheet at the nip of said rollers, then stopping also the second sheet at said nip, stapling and discharging the sheets already stacked on the stapling tray while the succeeding sheets are stopped as explained before, and sending two sheets together by said rollers to the stapling tray. In the above-explained structure, in order to superpose the sheets in the order of supply from the copying machine, the sheet-stopping portion of the transport path is curved in U-shape and the first sheet is maintained in close contact with the outside guide member of the curved portion thereby preventing the succeeding sheet from entering between the guide member and the first sheet.
However, such U-curved transport path requires a large space, eventually increasing the dimension of the apparatus. Also for achieving close contact of the first sheet with the output guide member of curved portion the sheet is required to have certain rigidity so that the usable sheets are limited. Besides the rigidity of sheet is variable depending on the temperature and humidity, and the function of the structure is not stable as the sheet may not be contacted with the guide member even for a same sheet. Also the mutual friction of sheets may generate static electricity, eventually disturbing the aligning operation on the stapling tray. Furthermore, since the sheet has to pass through the curved path even in the normal copying without stapling, the efficiency of transportation is low and the probability of sheet jamming is high.